Saying that intervening in foreign conflicts purely for humanitarian reasons is bad foreign policy is certainly legitimate -in fact, this is basically the position of the neocons who dominate the Bush administration (and recall that Bush himself opposed "national building" like NATO's eventual intervention in Yugoslavia after the spectacular failure of the UN in the 2000 election campaign). 

The Bush administration has never has been a unified monolith. There are and have been fissures. State and DOD were frequently at loggerheads when Powell was still there and CIA and Vice President’s office were not exactly friendly. Moreover, there are very real differences between 41 and 43. Philosophically, realists Scowcroft and Baker simply do not see eye to eye with neo cons such Wolfowitz and Douglas Feith. Before you interject I would certainly not accept Wolfowitz’s implication that what distinguishes the two camps is that one believes in democracy and the other in propping up tyrants. If I was to draw the line, I would say the main difference is how powerful each group believes the US to be.

Anyway, whereas you imply that Bush’s hostility to nation building in the 2000 campaign is continuous and compatible with post 911 rhetoric, I believe a major shift has occurred. After 911 the neo cons not only won over W, they won the battle for the Republican hearts and Republican minds. Their victory was so complete that in the lead up to the Iraq war realist critiques of the potential consequences of such an adventure went entirely unnoticed. The Republican base bought into Neo Con assumption that the UN was only ever a constraint on US power and rejected that realist belief that the multilateral institutions were a useful way of limiting potential costs and risks. Rumsfeld’s yammering about “old Europe”, "freedom Fries" etc were seen by the Republican base as evidence that Gulliver had at long last broken free of the Lilliputians. It was cause for celebration and no thought was given to the fact that such comments or campaigns might have caused the US a great deal of damage. Similarly, traditional realist laments about nation building went unnoticed for one simple reason. A key neo con assumption is that nation building, in the traditional sense, is not necessary. This is the main reason for the complete lack of planning in Iraq.  Society is organic.  Free it and the market from various “unnatural” restraints and it will flourish all by its own. The Republican base swallowed this assumption whole. With concerns over nation building out of the way, the Republicans were able to play up the humanitarian angle to a much greater extent then they would have had such a campaign occurred before 911 and in the process draw closer the liberal interventionists/ “liberal hawks”. (Sure the administration focused primarily on WMD, but Republican chattering classes, most notably those at the weekly Standard, and the right wing think tanks certainly pushed transformative power of democracy and the market line.)

For me whether Ignatieff can offer a convincing reply to this question is the litmus test that determines whether he is legitimately a liberal interventionist or merely an academic apologist for the Bush administration's imperial fantasies. 

Look there is always going to be a decent humanitarian case for following the Bush administration on any given adventure. After all, the type of places the Americans are limited to attacking openly are always going to tokens of world’s worst. That is why the rise of neo conservatism has made me dislike liberal interventionism a lot more than I used to. Not constrained by with usual realist laments, the project for new American Century promised to turn liberal interventionists into kids in a candy store. The Neo Cons insured that no one would be attacking them from the right flank at a time when post 911 zeitgeist in the States had muted left wing of the Democratic party. In return, these so called "liberal Hawks" did the intellectual heavy lifting for Bushco on the Iraq war.